1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
An image forming apparatuses using a laser has been in widespread use for electrophotographic image recording. Generally in this case, the image forming apparatus includes an optical scanning device which scans a drum (hereinafter, referred to as a “photosensitive drum”), which is photosensitive, by sending a laser light beam to the photosensitive drum using a light deflector in the axial direction of the photosensitive drum while rotating the photosensitive drum so that a latent image is formed on a surface of the photosensitive drum.
Recently, with respect to the image forming apparatus, in order to obtain a colored output image, a tandem type image forming apparatus having a plurality of (generally, four) photosensitive drums has been used. In addition, an image forming apparatus which has a high quality image and a small size has been highly in demand.
For example, Japanese Patent Application Laid-open No. 2007-010797 discloses an oblique incidence type optical scanning device for correcting the color differences in a color image caused by the occurrence of a scan line curvature due to a change in temperature. In the optical scanning device, a light beam which is deflected from the same deflecting and reflecting surface of a light deflector among light beams emitted from a plurality of light source units is incident from both sides in a sub-scanning direction interposing a normal line of a deflecting and reflecting surface. Further, an even number of reflecting mirrors in the sub-scanning direction are disposed on an optical path of the light beam, which is incident from one side in the sub-scanning direction with respect to the normal line of the deflecting and reflecting surface and is deflection-reflected, extending from the deflecting and reflecting surface to the corresponding scanned surface. Still further, an odd number of reflecting mirrors in the sub-scanning direction are disposed on an optical path of the light beam, which is incident from the opposite side in the sub-scanning direction with respect to the normal line of the deflecting and reflecting surface and is deflection-reflected, extending from the deflecting and reflecting surface to the corresponding scanned surface.
In addition, Japanese Patent Application Laid-open No. 2005-153347 discloses an optical scanning device for reducing the color difference without increasing an optical path length of a scanning optical system. The optical scanning device includes a plurality of optical housings arranged to be thermally insulated from each other. Each optical housing encloses a light source, a light deflecting unit having a deflecting and reflecting surface which deflects a light beam emitted from the light source, and a scanning optical system which focuses the light beam deflected by the light deflecting unit on a scanned surface. In the optical scanning device, the scanning directions of a plurality of light beams emitted from the optical housings are the same.
In addition, Japanese Patent Application Laid-open No. 2006-267398 discloses a scanning optical apparatus for reducing an interval of photosensitive members when a plurality of the photosensitive members are provided. In the scanning optical apparatus, a focusing optical unit is configured to include a plurality of reflecting mirrors and at least one lens having fθ characteristics. The reflecting mirrors reflect a light beam deflection-scanned from a rotating polygon mirror in a direction in which the light beam travels away from the photosensitive members and then cause the light beam to enter the photosensitive members. The lens is adapted to receive the light beam deflection-scanned from the rotating polygon mirror. The focusing optical unit for a plurality of light beams reflected from the same reflecting surface of the rotating polygon mirror is disposed at a position, including at position on the light beam, between the light beam, which has been passed through a gap between the rotating polygon mirror and the lens and has been reflected toward a photosensitive member, and the light beam, which has been reflected toward another photosensitive member, among a plurality of the light beams reflected from the same reflecting surface of the rotating polygon mirror.
In addition, Japanese Patent Application Laid-open No. 2008-299051 discloses an optical scanning device for configuring the whole apparatus in a compact size. In the optical scanning device, a light beam deflection-scanned by a deflecting plane is incident to at least one focusing optical device constituting a focusing optical system to pass through the focusing optical device; after the light beam is reflected by at least one reflecting optical device, the light beam is incident from the direction opposite to the incidence direction in an optical axis direction of the focusing optical system to pass through the focusing optical device; and at least two light beams deflection-scanned by a different deflecting plane of a deflecting unit pass through the focusing optical device again to intersect each other in a sub scan cross section.
However, in the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2007-010797, in the opposite scan type, the scan direction of one side optical system and the scan direction of the other side optical system are opposite to each other, and the direction of the position difference with respect to the main-scanning direction becomes the opposite direction. Therefore, although the timing to start writing (hereinafter, simply referred to as a “write starting timing”) is controlled based on an output signal of a synchronization sensor, in the case where the light incident to the synchronization sensor passes through the lens, the change in temperature in the one side optical system is not necessarily the same as the change in temperature of the other side optical system. Accordingly, although a deviation may occur between the write starting timing of the one side and the write starting timing of the other side, there is a problem in that a difference occurs. This negatively influences the quality of an image in the color difference in the main-scanning direction.
In addition, in the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2005-153347, in a case in which a write unit is divided into two write units, a temperature distribution in one unit or corresponding lens arrangement is not taken into consideration.
In addition, in the scanning optical apparatus disclosed in Japanese Patent Application Laid-open No. 2006-267398, there is a problem in that the scanning optical apparatus is configured to be thick. In addition, in the opposite scan type, the scan line curvatures of the one side and the scan line curvature of the other side are opposite to each other, so that there is a problem in that a color difference can occur.
In addition, in the optical scanning device disclosed in Japanese Patent Application Laid-open No. 2008-299051, since the scanning lens is disposed to be close to the polygon mirror, there is a problem in that the apparatus may be greatly influenced by a change in temperature.
There is a need to implement an apparatus which has small dimensions in both height and width and to reduce the deterioration in optical characteristics caused by a change in temperature.
Also, there is a need to implement a small-sized apparatus and to form a high quality image.